Ring illumination reflective elements on a generally planar surface

ABSTRACT

An inspection system and method uses a ring illumination apparatus to illuminate one or more reflective elements, such as solder balls on an electronic component or other protruding surfaces or objects. The ring illumination apparatus includes a substantially ring-shaped light source that provides a substantially even illumination across the one or more reflective elements. An illumination detection device detects light beams reflecting off of the illuminated reflective elements for forming a reflected image. A method of processing the reflected image includes locating one or more edges of each reflected image element representing an illuminated reflective element. The edges of the reflected image elements are located by determining the maximum intensity gradient in the pixels forming the reflected image element. The inspection system and method thereby determines various characteristics such as the absence/presence, location, pitch, size and shape of each reflective element.

RELATED APPLICATION

This application is a divisional of co-pending U.S. patent applicationSer. No. 08/807,397 filed on Feb. 26, 1997, entitled "INSPECTIONSYSTEM".

FIELD OF THE INVENTION

The present invention relates to inspection systems and methods and inparticular, to a system and method for inspecting surfaces and objectsusing a ring illumination apparatus.

BACKGROUND OF THE INVENTION

Digital data and signal processing techniques and technology havetremendously advanced the ability to use computers as data processingsystems to accomplish sophisticated inspection procedures without humanintervention. Almost every type of product can benefit from low cost,high precision, high speed inspection technology derived from these newdigital data and signal processing techniques.

For example, in computers and other electronic systems, the electricalconnections between electronic components ("chips") are critical to theoperation of the system. As a result of recent technological advances,electronic components are decreasing in size and increasing incomplexity, requiring a larger number of electrical connections to bemade in a smaller area. Inspection of the electronic components during amanufacturing process helps assure that electrical contacts are properlyformed and prevents failed electrical connections between electroniccomponents.

Semiconductor chips, for example, must be physically and electronicallyconnected to printed circuit boards using solder or flux betweenelectrical contacts on the chip and the circuit board. One type ofelectrical contact includes metal areas or pads on the semiconductorchip that must be electrically connected to corresponding metal areas orpads on the printed circuit board. Typically, small deposits of solderand/or flux are placed on the pads, heated and re-flowed, establishing amechanical and electrical connection between the corresponding pads.

A common soldering technique is to use preformed balls of solder thatare placed on the metal pads on the chip or substrate of an electroniccomponent, commonly known as a ball grid array (BGA). With the decreasein size of the electronic components and the increase in complexity, asmany as 400 or more solder balls must be precisely positioned in apredefined pattern on the chip or substrate to electrically connect thechip to the printed circuit board. During the process of positioning andadhering the solder balls to the metal pads on the chip or substrate, anumber of defects can occur that will detrimentally affect theelectrical connection between the chip and the printed circuit board.

If a solder ball does not sufficiently adhere to one of the pads, acritical electrical connection between the chip and the printed circuitboard could be lost. The misplacement of a solder ball can also resultin a failed connection and/or an electrical short circuit with anotheradjacent solder ball or metal pad. A solder ball that is malformed, toolarge or too small could also result in a defective electricalconnection even if properly positioned at the precise location on thepad.

Inspection of the solder balls is therefore critical to assure propersize and shape of the solder balls as well as precise placement andadherence of the solder balls to the appropriate pads on the printedcircuit board prior to establishing connections between the electroniccomponents. Inspection is also required for other electronic componentsrequiring precise electrical connections.

One prior art method of inspection is to have a human operator visuallyinspect each chip, printed circuit board or other electronic componentto detect defects in the solder balls or other electrical contacts.Manual inspection, however, is time-consuming, inaccurate, and a strainon human inspectors, particularly in light of the decreased size of theelectronic components and increased number of connections.

Video systems have also been used to inspect solder balls or othercontacts or features on electronic components. In such systems, a light,such as a ring light, illuminates the surface of the electroniccomponent to be inspected. A camera detects the light reflected from thesolder balls or contacts on the electronic component and the reflectedimage is displayed on a monitor.

The ring lights used in prior art inspection systems have been unable toprovide adequate illumination of solder balls on an electroniccomponent. One problem occurs when the ring lights do not provide lightbeams of sufficient intensity at outer regions of the area beinginspected and thus fail to illuminate some of the solder balls beinginspected, resulting in inaccurate determinations of theabsence/presence or position of the solder balls. Another problem existswhen a solder ball is only partially illuminated, preventing an accuratemeasurement of the true diameter and circularity of the solder ball.

Other inspection devices direct the light beams at a high angle withrespect to the chip, causing the light beams to reflect off themetalized pads, the substrate surface, or other substantially flatreflective surfaces that are not being inspected. In the resultingilluminated image detected by the video camera, the solder balls aredifficult to discern from the metal pads and other substantially flatreflective surfaces. This is a particular problem where the illuminatedimage is to be processed and analyzed by an image processor to detectthe absence/presence of solder balls and the condition of solder balls(e.g. location, diameter, and circularity).

Other ring lights direct light parallel to the surface of the componentbeing inspected and must be positioned against or around each electroniccomponent to obtain sufficient illumination of the entire surface of theelectronic component. If this type of ring light is not positionedagainst the surface of the component being inspected, the component willnot be sufficiently illuminated, particularly at the edges of thecomponent. This type of ring light must therefore be raised and loweredfor each individual electronic component to adequately illuminate eachelectronic component and does not allow a large number of electroniccomponents to be sequentially inspected quickly during a manufacturingprocess.

A further problem is that many prior art vision inspection systems stillrequire a human operator to examine the illuminated image of theelectronic component and detect defects such as missing, misplaced ormalformed solder balls. A visual inspection of the illuminated imagestill does not enable an accurate measurement of the size and shape ofthe solder balls.

Accordingly, a need exists for a system and method for inspecting solderballs or other reflective objects, surfaces or elements that adequatelyilluminates all of the reflective elements being inspected, allowingaccurate measurements and inspection of the reflective elements withoutilluminating other generally planar surfaces that are not beinginspected. There is also a need for a system and method that quickly andaccurately detects absence/presence of the illuminated reflectiveelements, determines their position, and measures the size and shape,e.g. the diameter and circularity of any protruding object, if desired.

SUMMARY OF THE INVENTION

The present invention features a ring illumination apparatus used in aninspection system for inspecting one or more reflective elements, suchas solder balls or other protruding reflective surfaces or objects, on asurface of an article disposed within a field of view of an illuminationdetection device. The ring illumination apparatus comprises asubstantially ring-shaped light source, for generating a plurality oflight beams. The light beams are directed toward the article in thefield of view in a range of angles of illumination that provides asubstantially even illumination of the article to be inspected acrossthe field of view such that any of the light beams striking thegenerally planar surfaces on the article are not reflected to theillumination detection device.

One embodiment of the substantially ring-shaped light source includes aplurality of light emitting elements, such as light emitting diodes(LEDs). In this embodiment, the light beams are directed in the range ofangles of illumination by mounting the light emitting elements so that acenter line of each of the plurality of light emitting elements isdisposed at a pre-defined angle, preferably about 4°, with respect tothe generally planar surface of the article. One type of light emittingelement includes LEDs having a beam spread in the range of about 20° to40° and emitting light beams having a far red spectral wavelength. Thisembodiment also preferably includes a light reflecting surface, such asa white surface, proximate to the plurality of light emitting elements.

The light emitting elements are preferably mounted to a mounting memberincluding an upper mounting portion and a side mounting portion. One orboth of the upper mounting portion and the side mounting portion has thelight reflecting surface. A light diffusing surface can also be disposedproximate to the light emitting elements.

According to another embodiment, ring illumination apparatus includes ahigh transmissivity, high diffusion diffuser, disposed proximate to thesubstantially ring shaped light source, for directing the light beams inthe desired range of angles of illumination with respect to the surfaceof the article to be inspected by angularly scattering the light beamsgenerated by the light source. According to this embodiment, the lightsource preferably includes a plurality of light emitting elements havinga centerline substantially parallel to the generally planar surface ofthe article. The high transmissivity, high diffusion diffuser preferablyincludes a diffusing film having a diffuse transmission of at least 85%.

The present invention also features an inspection system comprising: anillumination detection device, such as a camera, having a field of view;an article support surface, disposed underneath the ring illuminationapparatus, for supporting the article at a predetermined spacing fromsuch that the one or more reflective elements on the article are in thefield of view; and a ring illumination apparatus defining an aperturethrough which the field of view extends to the article to be inspected.The illumination detection device detects light beams reflected fromeach illuminated reflective element and forms a reflected imagerepresenting the illuminated reflective element. The inspection systemfurther includes a display, for displaying the reflected image, and animage processor, for processing the reflected image.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reading the following detailed description, takentogether with the drawings wherein:

FIG. 1 is a schematic view of an inspection system for inspecting one ormore surfaces or objects according to the present invention;

FIG. 2 is a top view of a ring illumination apparatus used to illuminatereflective elements in the inspection system and method according to oneembodiment of the present invention;

FIG. 3 is a side cross-sectional view of the ring illumination apparatusshown in FIG. 2 taken along line III--III;

FIG. 4 is a side cross-sectional view of the ring illumination apparatusaccording to another embodiment of the present invention;

FIG. 5 is a schematic view of a reflected image detected by theinspection system according to one embodiment the present invention;

FIG. 6 is a schematic view of a single reflected image element to beprocessed according to one embodiment of the present invention;

FIG. 7 is a flow chart of a method for processing a reflected imageaccording to the present invention;

FIG. 8 is a flow, chart of a method for locating an edge of a reflectedimage element in a processed reflected image according to the presentinvention; and

FIG. 9 is a flow chart of a method for determining a number of reflectedimage elements in a reflected image according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An inspection system 10, FIG. 1 (shown in an exaggerated perspectiveview for purposes of clarity), according to the present invention, isused to inspect one or more reflective elements, such as protrudingreflective surfaces or objects 12, disposed on an article 14. In oneexample, the protruding reflective objects 12 are disposed on agenerally planar surface 18 that has both reflective and non-reflectiveareas. The article 14 typically includes an array of protrudingreflective objects 12 made of metal or other light reflective materials.

In the exemplary embodiment, the inspection system 10 is used to inspectan array of solder balls disposed on metal pads on a chip or othersubstrate of an electronic component, such as BGA or micro BGAsemi-conductor packages, chip scale packaging (CSP), or flex circuits.The positioning, size and shape of solder balls are inspected tofacilitate proper electrical connection between the chip and otherelectronic components, such as printed circuit boards. The presentinvention, however, contemplates inspecting any type or shape ofreflective elements including, but not limited to, protrusions,deviations, and other contoured surfaces or objects on an article,arranged in any pattern on any type of article.

The present inspection system 10 includes a field of view 16 that coversthe protruding reflective objects 12 disposed on the article 14, and aring illumination apparatus 20 defining an aperture 22 through which thefield of view 16 extends. The ring illumination apparatus 20 includes asubstantially ring-shaped light source 24 that generates light beams 26and directs the light beams 26 into the field of view 16 on the article14 such that the protruding reflective objects 12 are illuminated. Thelight beams 26 preferably provide a substantially even intensity oflight across the field of view 16 on the article 14 and a substantiallyeven illumination of all of the protruding reflective objects 12 in thefield of view 16, as will be described in greater detail below. Althoughshown as a generally circular ring light, ring illumination apparatus 20may also be in the shape of an oval or other similar shape.

The inspection system 10 further includes an illumination detectiondevice 30, such as a CCD camera, disposed above the ring illuminationapparatus 20, for example, at about 14 inches above the ringillumination apparatus 20. The illumination detection device detectslight beams 32 reflected from each protruding reflective object 12 andcreates a reflected image. One example of an illumination detectiondevice 30 is a CCD camera having a resolution of about 640×480 pixels;although the present invention contemplates other types of cameras anddevices capable of detecting an illuminated image.

The substantially ring-shaped light source 24 directs light beams 26 atangles of illumination with respect to the article 14 that causenon-detected light beams 34 to reflect from the flat or planar surfaces18 on the article 14 outside of the field of view 16 or range of theillumination detection device 30. The preferred angles of illuminationof the light beams 26 are in a range of less than or equal to about 10°and are provided by different embodiments of the substantiallyring-shaped light source 24, as will be described in greater detailbelow. Accordingly, the detected light beams 32 reflecting from thereflective objects 12 create the reflected image and the non-detectedlight beams 34 reflecting from flat surfaces 18 are not included in thereflected image.

The ring illumination apparatus 20 can be mounted to a mounting support28, for example, with a mounting bracket 29. The mounting support 28 andmounting bracket 29 support the ring illumination apparatus 20 in thedesired position, allowing the article 14 to be disposed or positionedwith the reflective objects 12 in the field of view 16. An articlesupport surface 27 disposed beneath the ring illumination apparatus 20supports the article to be inspected 14 so that the reflective surfaces12 are in the field of view 16. In one example, the article supportsurface 27 is moved to index articles 14 successively into the field ofview 16 for inspection during a manufacturing process, as is well knownin the art. Alternatively, the ring illumination apparatus 20 andillumination detection device 30 are indexed or moved over each article14 being inspected. When the articles 14 and/or ring illuminationapparatus 20 and illumination detection device 30 are moved with respectto one another during inspection, the light source 24 preferably uses astrobed power supply that eliminates the effects of motion.

The inspection system 10 further includes an image processor 38 thatprocesses the reflected image and determines inspection informationincluding, but not limited to, the absence/presence, location, pitch,size, and shape of each protruding reflective surface 12, as will bedescribed in greater detail below.

The inspection system 10 optionally includes a monitor 36 that allowsthe reflected image to be viewed by an operator. The monitor 36facilitates the visual inspection and alignment of the reflectiveobjects 12 by the operator. The present invention also contemplatesother output or peripheral devices including, but not limited to, aprinter or storage device. The image processor 38 can transmit theinspection information to the monitor 36 (if provided) for viewing bythe operator or to other peripherals or devices, such as by digital I/O,RS-232 serial communication or Ethernet networking.

The preferred embodiment of the ring illumination apparatus 20, FIG. 2,includes a mounting member 40 that defines the aperture 22 and isdisposed above the article to be inspected 14. The mounting member 40preferably includes one or more fastener receiving regions 42, forbolting or otherwise fastening to mounting bracket 29. The mountingmember 40 can also include a power cord receiving region 44 thatreceives a power cord connected to a power source (not shown), forpowering the light source 24. Although the exemplary embodiment uses astrobed power supply that eliminates the effects of motion on theillumination of articles 14, the present invention contemplates any typeof power source.

The preferred embodiment of the substantially ring-shaped light source24 includes a plurality of light emitting elements 50, such as lightemitting diodes (LEDs), mounted to the mounting member 40 in asubstantially ring shape. One example of each light emitting element 50includes an LED that emits light beams having a far red spectralwavelength (e.g. about 660 nanometers) and a beam spread α ofapproximately 26° to 28°, such as a TLRA 155BP LED made by Toshiba. CCDcameras respond well to far red LED's which allow the effect of ambientlight to be filtered out and substantially eliminated during theinspection process. In the exemplary embodiment, about sixty (60) ofsuch LEDs 50 are mounted in a ring around the mounting member 40. Thepresent invention contemplates any type and number of light emittingelements that provide the desired even illumination across the field ofview.

According to one embodiment of the substantially ring-shaped lightsource 24, FIG. 3, the light beams 26 are directed at the article 14 inthe desired range of angles of illumination by mounting the lightemitting elements 50 so that a central axis or center line 54 of eachlight emitting element 50 is disposed at an angle θ with respect to aplane 52 parallel to the generally planar surface 18 of the article 14.A preferred angle θ of about 4° provides light beams 26 with a low angleof illumination (e.g. less than or equal to about 10°) onto the planarsurface 18 of the article, such that the planar surface 18 (eithernon-reflective or reflective) will either not reflect the light beams 26or will reflect the light beams 26 as non-detected light beams 34 thatextend outside field of view 16 and are therefore not detected by theillumination detection device 30. This embodiment preferably includes alight reflecting surface 64, such as white paint or a reflectivecoating, proximate to each light emitting element 50 and may alsoinclude a light diffusing surface 66 generally in front of the lightemitting elements 50, for scattering light and directing light moreevenly across the article 14.

According to known light physics principles of reflectivity, when lighthits a reflective surface, the angle of reflection is equal to the angleof incidence, measured from the axis perpendicular to the tangent of thereflective surface. If the angle θ of the light emitting element 50 istoo large, the light emitting elements 50 will provide light beams 26with a high angle of incidence and therefore a high angle of reflection,causing the light beams 26 to reflect off the planar surfaces 18 towardsthe illumination detection device 30.

Lowering the mounting angle θ of the light emitting elements 50therefore lowers the angle of illumination of light beams 26 such thatlight beams 26 reflect from planar surfaces 18, such as reflectivemetalized pads and non-reflective flat surfaces, at a lower angle ofreflection outside field of view 16 as non-detected light beams 34 thatare not detected by detection device 30. The light beams 26 that hit theprotruding reflective objects 12, on the other hand, will reflectthrough the aperture 22 to the illumination detection device 30 asdetected light beams 32. The protruding reflective objects 12 arethereby illuminated for inspection while the planar surfaces 18 that arenot being inspected are not illuminated.

In this embodiment, the angle θ of the light emitting elements 50 isalso preferably greater than zero to allow sufficient spacing S betweenthe ring illumination apparatus 20 and the article 14 being inspectedwhile ensuring an even illumination across the entire field of view 16on the article 14. The spacing S of the ring illumination apparatus 20from the surface 18 allows articles 14 to be passed beneath the ringillumination apparatus 20 into and out of the field of view 16, e.g.,when inspecting during a manufacturing process. The preferred spacing Sis as small as possible without interfering with the article 14 passingbeneath the ring illumination apparatus 20 during the inspectionprocess, and typically in the range of about 1/4 to 1/2 in. The lightemitting elements 50 are also preferably positioned as close asphysically possible to the bottom region 56 of the ring illuminationapparatus 20.

The central light beam area 25 of light beam 26 directed along thecenter line 54 of each light emitting element 50 typically has thehighest power or intensity. An angle θ of approximately zero (0) degreeswill result in the central light beam area 25 being directedsubstantially parallel to the article 14. When the parallel centrallight beam area 25 is spaced from the article 14, the edges 17 of thefield of view 16 on the article 14 will only receive lower power orintensity light beams, and protruding reflective objects 12 locatedproximate to the edges 17 of the field of view 16 may not besufficiently illuminated.

By directing the central light beam area 25 towards the opposite sidesor edges 17 of the field of view 16 on the article 14, a substantiallyeven intensity of light beams is provided across the entire field ofview 16 to provide a substantially even illumination of every protrudingreflective object 12 located in the field of view 16. The angle θ oflight emitting elements 50 is therefore defined so that an imaginaryline extending from the center line 54 generally intersects or overlapsthe opposite edges or sides 17 of the field of view 16, but withoutdirecting light beams 26 at an angle of illumination high enough tocause detection of light beams reflected from the planar surfaces 18.

The substantially ring-shaped light source 24 preferably forms adiameter d, e.g., measured from the front portion of the light emittingelements 50, of approximately 2.5 to 3 times a dimension or width of thefield of view 16 on the article 14. This preferred diameter d allows thecenter light beam area 25 to be directed to the edges 17 with a lowangle of illumination while maintaining sufficient spacing S between thering illumination apparatus 20 and the article 14. In one example, adiameter of approximately 5.5 inches is used to evenly illuminate afield of view 16 on an article 14 of approximately 2 in. by 2 in.Accordingly, the diameter d of the ring-shaped light source 24 as wellas the angle θ of the light emitting elements 50 allow the light beams26 to provide even illumination across the field of view 16 on thearticle 14, while preventing illumination of unwanted planar surfaces 18and allowing articles 14 to be passed beneath the ring illuminationapparatus 20.

The preferred embodiment of the ring illumination apparatus 20 furtherincludes an upper mounting portion 60 forming the aperture 22 and a sidemounting portion 62 extending from the upper mounting portion 60. In theexemplary embodiment, the plurality of light emitting elements 50 aremounted to the side mounting portion 62 which is shaped as a ring and iswelded or otherwise secured to the upper mounting portion 60. Either thelight emitting elements 50 or the side mounting portion 62 can be angledto provide the angle θ.

The light diffusing surface 66 can be formed as a light diffusing memberor ring mounted to the upper mounting portion 60. The present inventioncontemplates other types of surfaces that diffuse or scatter the lightfrom the light source, such as a light diffusing surface directlydisposed on each individual light emitting element 50.

The light reflecting surface 64, such as white paint or other lightreflecting colors, can be provided on the side mounting portion 62 andupper mounting portion 60. In one embodiment, the diameter of theaperture 22 is approximately 70 to 80 percent the diameter d of thesubstantially ring-shaped light source 24 such that a portion 65 of theupper mounting portion 60 extends beyond the light emitting elements 50and has a light reflecting surface 64.

In accordance with another embodiment of the ring illumination apparatus20a, FIG. 4, the light beams 26 are directed at the article 14 in thedesired range of angles of illumination with a high transmissivity, highdiffusion diffuser 66a that disperses and angularly scatters light beamsas they are emitted from the light emitting elements 50a. The angularlyscattered light beams 26a provide the substantially even intensity oflight across the field of view 16 on the article 14 while preventingillumination of the flat reflective surfaces and allowing the spacing Sfrom the ring illumination apparatus 20a. In this embodiment, the angleθ of the of the light emitting elements 50 can be eliminated and ispreferably in a range of 0° to 8°.

When the high transmissivity, high diffusion diffuser 66a is used, alight reflecting surface 64 is not provided on the portion 65a of theupper mounting portion 60a that extends beyond the diffuser 66. Thisportion of the upper mounting portion 65a can have a black or othernon-reflective surface or can be eliminated entirely.

The high transmissivity, high diffusion diffuser 66a has a diffusetransmission of about 85% or more and preferably in the range of about88% to 90%. One type of high transmissivity, high diffusion diffuser 66ais an acrylic Diffusing Film Alternative (DFA) manufactured by 3M™. Thepresent invention also contemplates other suitable high transmissivity,high diffusion films that provide the desired diffuse transmission andthe desired angular scattering of the light beams.

A larger ring light apparatus can be used for larger fields of view. Fortighter applications, a smaller ring light can be used with conicalmirrors that fold the optical path to preserve the internal light pathand direct the light beams at the article in the desired angle ofillumination range.

The method of using the inspection system 10 to inspect one or morereflective elements, such as reflective objects 12, disposed on thearticle 14 includes positioning the article 14 in the field of view 16.Either a series of articles 14 are sequentially passed or indexedthrough the field of view 16 beneath the ring illumination apparatus 20or the ring illumination apparatus 20 is indexed over each article 14. Aring of light beams 26 is directed in a desired range of angles ofillumination from the ring illumination apparatus 20 toward thereflective element(s) 12 in the field of view 16 containing, to providea substantially even intensity of light beams across the field of view16 without illuminating planar surfaces 18 on the article 14.

The system and method of the present invention detects light beamsreflected from the illuminated reflective elements, such as protrudingreflective objects 12, to form a reflected image 70, FIG. 5, of theilluminated reflective elements. The reflected image includes reflectedimage elements 72 representing the illuminated reflective elements, suchas reflective objects 12. The reflected image 70 is acquired byconverting the analog output signal of the illumination detection device(camera) 30 into a plurality of digital signals, each representing asmall picture element or pixel of the image. The digital signals formingthe reflected image 70 can then be converted to analog signals fordisplay on the monitor 36 and/or processed by the image processor 38(see FIG. 1).

The reflected image 70 is processed to determine inspection informationincluding, but not limited to, absence/presence, location, size, andshape of the reflective elements. In the exemplary system and method,which is not intended to limit the present invention, the inspectionsystem 10 is used to inspect an array of solder balls disposed on solderpads on an electronic component, such as a semiconductor chip. In thereflected image 70, each solder ball appears as a reflected imageelement 72, FIG. 6, having a "doughnut" or ring shape. The inspectioninformation pertaining to the array of solder balls includes, but is notlimited to, absence/presence of each solder ball in the array, locationof each solder ball, the pitch between solder balls, malformed solderballs, the diameter of each solder ball, and the circularity of eachsolder ball.

The present invention also features a method 100, FIG. 7, of processinga reflected image 70 including a pattern of reflected image elements 72.The method of processing the reflected image includes locating thepattern of reflected image elements 72 in the reflected image 70, step110; fitting an outline to each reflected image element 72 in thepattern of reflected image elements, step 120; and determininginspection information pertaining to the reflective elements representedby the reflected image 70, step 130.

One way of locating the pattern of reflected image elements 72, step110, is by identifying a group of reflected image elements 72, forexample, by creating a window 75 (FIG. 5) around an outside row or groupof reflected image elements 72. The outside edge 76 of each reflectedimage element 72 in the group is then located. One or more lines 74 arefit to the outside edges 76 of the reflected image elements 72 (see FIG.5) to locate the pattern and determine the "expected" location of eachreflected image element 72 in the pattern. Although the exemplaryembodiment shows a rectangular grid of image elements 72, the presentinvention also contemplates locating a circular or other pattern ofreflected image elements. In a circular pattern, the outside edges ofthe image elements formed in a circle are located and a circle fitalgorithm is used to fit a circle to the outside edges and to locate thepattern.

Another way of locating the reflected image is by locating referencemarks or fiducials 73 disposed in predetermined locations with respectto the reflected image elements 72 in the reflected image 70. A furtherway of locating the reflected image is by correlating a template ormodel of a known pattern with the reflected image.

Once the pattern of the reflected image 70 is located, the outline 77 isfit to each reflected image element, step 120, for example, by creatinga window 78 (FIG. 6) around each reflected image element 72 at eachexpected location and locating points 76a-76d on the edge 76 of thereflected image element 72. For a reflected image element 72 having acircular or ring shape, at least three of the points 76a-76d are neededto fit the circular outline 77 and four points 76a-76d are needed todetermine the circularity of the outline 77 and corresponding reflectiveelement. The points 76a-76d are used to fit or define the circularoutline 77. In the preferred embodiment, eight (8) or more points arelocated, and the locations of the points are fed to a circle fitalgorithm which accurately determines the size and circularity of thereflective elements.

The preferred method 200, FIG. 8, of locating the outside edge 76 orfour points 76a-76d, for example, on an edge of each reflected imageelement 72 includes dividing the reflected image 70 into a plurality ofpixels having a gray scale value corresponding to an intensity level ofreflected light in the reflected image 70, step 210. In one example,each pixel is represented by eight (8) bits with a gray scale value ofzero (0) being the darkest pixel and a gray scale value of 255 being thebrightest pixel. A series of pixels is examined for each reflected imageelement 72 within the window 75.

Vectors 79a-79d are used to find points 76a-76d. The vectors 79a-79d arepositioned to intersect the expected location of the image element 72.Along the path of each vector 79a-79d, an intensity gradient at eachpixel is determined, step 220, by differentiating between the gray scalevalues of pixels on either side of each pixel. The point of maximumgradient (i.e. the steepest or greatest change from darkest to brightestpixels) is located and is assigned to correspond to the edge 76 of thereflected image element 72, step 230. The preferred method includestaking the highest intensity gradients for each reflected image element72 and fitting an ellipse to the highest intensity gradients. The peakof the ellipse corresponds to the edge to be located with sub-pixelaccuracy, thereby allowing a more accurate calculation of the dimensionsand shape of the reflective surfaces represented by each reflected imageelement 72.

Since the method above processes the reflected image elements 72according to their expected locations as determined by fitting the lines74 along the edges 76 (FIG. 5), an extra or additional reflected imageelement 72a corresponding to an added solder ball or other reflectivesurface may not be detected. The present image processing method furtherincludes a method 300, FIG. 9, of determining the number of reflectedimage elements 72. Determining the number of reflected image elements 72in the entire reflected image 70, not just at the expected locations,allows the absence/presence of solder balls or other reflective surfacesto be easily determined.

The number of reflected image elements 72 is determined by dividing thereflected image 70 into a plurality of pixels having a gray scale valuecorresponding to the intensity of light in the reflected image 70, step310. Groups of pixels having a gray scale value above a threshold valueare then located, step 320, and the number of groups of pixels(corresponding to the number of reflected image elements 72) arecounted, step 330. Determining the number of reflected image elements 72allows a determination of missing, misplaced or extra reflectiveelements, such as solder balls or other reflective objects.

The image processing method of the present invention also includes acalibration process that can be performed to allow the inspectioninformation measurements to be expressed in conventional units and tocorrect for magnification, perspective errors, and other effects. Thecalibration procedure involves measuring a target of known dimensions,for example, an array of dots having known sizes and known locations onthe target. The relationship between the coordinates of the target imageas determined by the image processor and the known location of the dotson the target are calculated to determine the correlation between pixelsand conventional units. The present method also contemplates heightcorrection of the part being inspected relative to the calibrationtarget to account for errors caused by optical magnification.

The present method for processing the reflected image 70 used togetherwith the ring illumination apparatus 20 described above provides a moreaccurate determination of the diameter and circularity of solder ballson an electronic component or other article. Directing the light beamsin the desired range of angles of illumination e.g. by angling the lightsource 24 at an angle θ, by using a high diffusion diffuser, or by usingconical mirrors, illuminates a portion of the top surface of each solderball so that the point of maximum gradient corresponds to a knownpercentage of the true diameter of the solder ball, e.g. about 70%.Directing the light beams in the desired range of angles of illuminationalso prevents illumination of solder pads or other generally planarsurface areas from interfering with the determination of the maximumgradient.

When the ring illumination apparatus of the present invention is usedtogether with the present method of processing the reflected image, themeasurements made during the inspection have a high degree of accuracyand repeatability. The present invention, however, contemplates usingthe ring illumination apparatus with other methods for processing thereflected image as well as using this method of processing the reflectedimage with another type of illumination apparatus.

Accordingly, the present invention provides an inspection system andmethod that accurately inspects and measures one or more surfaces orobjects, such as solder balls, protrusions, intrusions, deviations,concavities, and other reflective elements on an article. The inspectionsystem and method evenly illuminates all of the surfaces or objects tobe inspected without concern for illuminating flat or unwanted surfaceson the article or electronic component. The inspection method alsoincludes an image processing method that more accurately measures thesize and shape of individual reflective elements or solder balls.

Modifications and substitutions by one of ordinary skill in the art areconsidered to be within the scope of the present invention which is notto be limited except by the claims which follow.

What is claimed is:
 1. A ring illumination apparatus, for illuminating aplurality of reflective elements disposed on a generally planar surfaceof an article to be inspected and disposed within in a field of view ofan illumination detection device, said ring illumination apparatuscomprising:a substantially ring-shaped light source having a predefineddiameter, said substantially ring-shaped light source including aplurality of light emitting elements, for generating a plurality oflight beams, wherein each of said plurality of light emitting elementsare mounted at a predefined angle with respect to a plane parallel tosaid generally planar surface of said article, such that a centerline ofeach of said plurality of light emitting elements is disposed at saidpredefined angle with respect to said generally planar surface of saidarticle to be inspected; a light reflecting surface disposed generallybehind each of said plurality of light emitting elements, for reflectingat least some of said light beams generated by said light emittingelements; a light diffusing surface disposed generally in front of eachof said plurality of light emitting elements, for diffusing at leastsome of said plurality of light beams as said plurality of light beamsare directed toward said article within said field of view; and whereinsaid plurality of light beams are directed toward said article in saidfield of view in a range of angles of illumination with respect to saidgenerally planar surface of said article such that said plurality ofreflective elements on said article are illuminated simultaneously andany of said plurality of light beams striking said generally planarsurface of said article to be inspected are not reflected to saidillumination detection device.
 2. The ring illumination apparatus ofclaim 1 wherein said plurality of reflective elements include an arrayof solder balls disposed on an electronic component.
 3. The ringillumination apparatus of claim 1 wherein said predefined diameter andsaid predefined angle are defined such that a central light beam area ofeach of said plurality of light beams is directed across said field ofview toward an opposite edge of said field of view on said article to beinspected.
 4. The ring illumination apparatus of claim 1 wherein saidlight reflecting surface includes a white surface proximate to saidlight emitting elements.
 5. The ring illumination apparatus of claim 1further including a mounting member, said plurality of light emittingelements being mounted to said mounting member in a substantially ringshape.
 6. The ring illumination apparatus of claim 5 wherein saidmounting member includes an upper mounting portion and a side mountingportion, said plurality of light emitting elements being mounted to atleast one of said upper mounting portion and said side mounting portion,at least one of said upper portion and said side portion having a lightreflecting surface.
 7. The ring illumination apparatus of claim 1wherein said plurality of light emitting elements are light emittingdiodes (LEDs).
 8. The ring illumination apparatus of claim 7 whereineach of said plurality of LEDs generates a light beam which spreads in arange of about 20° to 40°.
 9. The ring illumination apparatus of claim 7wherein each of said plurality of LEDs emits light beams having a farred spectral wavelength.
 10. The ring illumination apparatus of claim 1wherein said predefined angle of said light emitting elements is about4°.
 11. The ring illumination apparatus of claim 1 wherein saidpredefined diameter of said substantially ring-shaped light source is ina range of about 21/2 to 3 times a dimension of said field of view atsaid article to be inspected.
 12. A ring illumination apparatus, forilluminating a plurality of reflective elements disposed on a generallyplanar surface of an article to be inspected and disposed within in afield of view of an illumination detection device, said ringillumination apparatus comprising:a substantially ring-shaped lightsource having a predefined diameter, said substantially ring-shapedlight source generating a plurality of light beams, said substantiallyring-shaped light source including:a plurality of light emittingelements, mounted at a predefined angle with respect to a plane parallelto said generally planar surface of said article, for directing saidplurality of light beams generated by said substantially ring-shapedlight source toward said article in said field of view, wherein saidpredefined diameter and said predefined angle are defined such that acentral light beam area of each of said plurality of light beams isdirected across said field of view toward an opposite edge of said fieldof view on said article to be inspected such that said plurality oflight beams provide a substantially even illumination of each of saidplurality of reflective elements in said field of view on said articleto be inspected without being reflected from said generally planarsurface to said illumination detection device.
 13. The ring illuminationapparatus of claim 12 wherein said predefined angle is about 4°.
 14. Thering illumination apparatus of claim 13 wherein said predefined diameteris about 21/2 to 3 times a dimension of said field of view on saidarticle to be inspected.
 15. The ring illumination apparatus of claim 12further including a light reflecting surface disposed generally behindeach of said plurality of light emitting elements, for reflecting atleast some of said light beams generated by said light emittingelements.
 16. The ring illumination apparatus of claim 15 furtherincluding a light diffusing surface disposed generally in front of eachof said plurality of light emitting elements, for diffusing at leastsome of said plurality of light beams as said plurality of light beamsare directed towards said article within said field of view.
 17. Thering illumination apparatus of claim 16 wherein said light reflectingsurface includes a white surface proximate said light emitting elements.18. The ring illumination apparatus of claim 12 wherein said pluralityof light emitting elements are light emitting diodes (LEDs).
 19. Thering illumination apparatus of claim 18 wherein each of said pluralityof LEDs generates a light beam which spreads in a range of about 20° to40°.
 20. The ring illumination apparatus of claim 19 wherein each ofsaid plurality of LEDs emits light beams having a far red spectralwavelength.
 21. A ring illumination apparatus, for illuminating at leastone reflective element disposed on a generally planar surface of anarticle to be inspected and disposed within a field of view of anillumination detection device, said ring illumination apparatuscomprising:a substantially ring shaped light source including aplurality of light emitting diodes (LEDs), for generating a plurality oflight beams each having a beam spread in a range of about 20° to 40° andhaving a far red spectral wavelength, wherein each of said plurality ofLEDs are mounted at a predefined angle of about 4° with respect to aplane parallel to said generally planar surface of said article andforming a ring having a predefined diameter of about 2.5 to 3 times adimension of said field of view on said article to be inspected; a lightreflecting surface disposed generally behind each of said plurality ofLEDs, for reflecting at least some of said light beams generated by saidLEDs; and a light diffusing surface disposed generally in front of eachof said plurality of LEDs, for diffusing at least some of said pluralityof light beams as said plurality of light beams are directed towardssaid article within said field of view.
 22. A method of illuminating anarticle having a plurality of reflective elements disposed on agenerally planar surface, said method comprising the steps of:placingsaid article with said plurality of reflective elements within a fieldof view of an illumination detection device and beneath a ringillumination apparatus; and directing a plurality of light beams at saidarticle using said ring illumination apparatus such that a central lightbeam area of each of said plurality of light beams is directed acrosssaid field of view toward an opposite edge of said field of view on saidarticle to be inspected, wherein said plurality of light beams aredirected toward said article being inspected at an angle of illuminationwith respect to said article being inspected such that said light isreflected from each of said plurality of reflective elements toward saidillumination detection device and said light is reflected from saidgenerally planar surface away from said illumination detection device.23. The method of claim 22 wherein said article includes a substratehaving said generally planar surface and a plurality of roundedreflective elements disposed thereon.
 24. The method of claim 22 whereinsaid angle of illumination is about 4°.
 25. The method of claim 24wherein a diameter of said ring illumination apparatus is about 2.5 to 3times a dimension of said field of view on said article to be inspected.26. The method of claim 22 wherein each of said plurality of light beamshave a beam spread of 20° to 40°.
 27. The method of claim 22 whereineach of said plurality of light beams have a far red spectralwavelength.